1. Field of the Invention
This invention relates to a process for electroplating the surface of thermoplastic resin articles and new filled nylon-6 resins for use therein. The resulting metal plated thermoplastic articles have both excellent adhesion between the metal and the thermoplastic resin as well as excellent appearance of the finished metal surface.
2. Discussion of the Prior Art
The demand for low cost metal plated plastic articles has been rapidly increasing. These metal plated articles can have either a metallic function, metallic appearance or both, and are used in such industries as automotive, home, appliance, radio and television, decorative containers, etc.
Before being plated, the surface of the plastic articles have to be treated to render them electrically conductive and also to modify them so that the electrodeposited metal adheres well thereto.
Basic to the conventionally known plastic plating processes are the steps of: A. Preconditioning the surface of the article by chemical etching or mechanical roughening in order to promote bonding with the coatings to be applied later. The chemical etchant is generally a strong oxidizing acid etch containing, for example, chromic acid with sulfuric or phosphoric acid. B. Sensitizing and activating the surface by applying a metallic catalyst usually by depositing the metal from a solution of its salts by chemical reduction. This is accomplished by immersing the substrate in a solution of stannous chloride or other source of stannous ions followed by immersion in a solution of palladium chloride or silver nitrate or immersing the substrate in a colloidal catalyst solution comprised of stannous and palladium ions followed by treatment with an acid accelerator to deposit the palladium. C. Electroless metal deposition of copper or nickel by immersing the substrate in an electroless copper or nickel containing bath. D. Electrolytically plating copper, nickel, chromium, or combinations thereof.
An important factor affecting the electroless metal coverage, adhesion, and appearance of the final product is step A (above); preconditioning the surface. If the surface of the substrate is not sufficiently etched, good metal to substrate adhesion is not obtainable. Excessive etching, on the other hand, will not promote good adhesion, but the surface of the final product will have a poor appearance. By "final product" we mean the electroplated article.
Electroplating processes are known in the art. For example, Japanese Pat. No. 50 14,676, discloses the plating of polyamides by substituting either a 7 to 18 wt% HCL or 20 to 40 wt% H.sub.2 SO.sub.4 solution for the chromic acid/sulfuric acid etch more commonly used. However, there is no discussion regarding the quality of the finish obtained.
Japanese Pat. No. 75 51,426 teaches the plating of unsaturated polyester thermoset resins, wherein the resins are filled with a filler component comprised of 16 parts per hundred resin (phr) TiO.sub.2, 35 phr SiO.sub.2 and 60 phr CaO. A 30% HCL solution is used as the etchant. However, the silica filler is not suitable for attack by dilute acid solutions and CaO is highly alkaline and could have an adverse effect on the molecular weight of thermoplastics, such as the polyamides during their high temperature processing and compounding.
U.S. Pat. No. 3,688,017 describes an improved surface activating step for polyamide articles during an electroplating process. The article is briefly exposed to an aqueous solution of a reducing acid to ionize surface groups, in order to improve receptivity and adhesion of the nylon surface for metal plating. The process requires thorough washing of the treated surface prior to treatment with noble metal catalysts.